Determinants of Endurance Exercise Capacity in the Heat in Prepubertal Boys

Abstract

The goal of this study was to identify factors which limit exercise endurance in hot ambient conditions in prepubertal boys. Eight healthy non-acclimatized, highly physically active prepubertal boys performed steady load cycling at approximately 65 % peak V·O₂ to exhaustion in both cool (19.6 ± 0.6 °C, 66.4 ± 11.0 % relative humidity) and hot (31.0 ± 0.3 °C, 56.9 ± 2.0 % relative humidity) environmental conditions. Cardiac output, oxygen uptake, rectal temperature (T_re), rating of perceived exertion (RPE), blood pressure, and calculated arterial venous oxygen difference were obtained serially in each testing session, and percent dehydration was calculated from body weight loss. Endurance time was significantly shorter in the hot condition (29.30 ± 6.19 minutes versus 41.38 ± 6.30 minutes in the cool room). No significant differences in circulatory markers or hydration status were observed either during testing or between cycling thermal conditions. Rate of rise of T_re was greater during exercise in the heat, but no significant difference in T_re between conditions was observed at exhaustion. Mean values of RPE were consistently greater during exercise in the heat, but these differences did not reach statistical significance. These findings support the concept that rises in core temperature and/or brain perception (RPE) rather than circulatory insufficiency may be the critical factors defining limits to exercise in the heat.

Thermoregulation - exercise testing - heat exhaustion - physical fitness - children

References

• 1 Bar-Or O. Temperature regulation during exercise in children and adolescents. Gisolfi CV, Lamb DR Perspectives in Exercise Science and Sports Medicine. Vol. 2. Indianapolis; Benchmark Press 1988: 199-226
• 2 Cheung S S, McLellan T M. Influence of heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress.  J Appl Physiol. 1988;  84 1731-1739
• 3 Cheung S S, Sleivert G G. Multiple triggers for hyperthermic fatigue and exhaustion.  Exerc Sports Sci Rev. 2004;  32 100-106
• 4 Cooper T, Willman V L, Hanlon C R. Cardiac and peripheral vascular responses to hyperthermia induced by blood stream heating.  J Thor Cardiovasc Surg. 1962;  44 667-673
• 5 Coyle E F, Gonzalez-Alonso J. Cardiovascular drift during prolonged exercise: new perspectives.  Exerc Sport Sci Rev. 2001;  29 88-92
• 6 Drinkwater B L, Kepprat I C, Denton J E, Crist J L, Horvath S M. Response of prepubertal girls and college women to work in the heat.  J Appl Physiol. 1977;  43 1046-1053
• 7 Eston R G, Lamb K L, Bain A, Williams A M, Williams J G. Validity of a perceived exertion scale for children: a pilot study.  Percept Mot Skills. 1994;  78 691-696
• 8 Febbraio M A. Does muscle function and metabolism affect exercise performance in the heat?.  Exerc Sports Sci Rev. 2000;  28 171-176
• 9 Fuller A, Carter R N, Mitchell D. Brain and abdominal temperatures at fatigue in rats exercising in the heat.  J Appl Physiol. 1998;  84 877-883
• 10 Galloway S DR, Maughan R J. Effects of ambient temperature on the capacity to perform prolonged exercise in man.  J Physiol. 1995;  489 35-36
• 11 Gonzalez-Alonso J, Calbet J A, Nielsen B. Muscle blood flow is reduced with dehydration during prolonged exercise in humans.  J Physiol. 1998;  513 895-905
• 12 Gonzalez-Alonso J, Mora-Rodriguez R, Below P R, Coyle E F. Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise.  J Appl Physiol. 1995;  79 1487-1496
• 13 Gonzalez-Alonso J, Mora-Rodriguez R, Below P R, Coyle E F. Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise.  J Appl Physiol. 1997;  82 1229-1236
• 14 Gonzalez-Alonso J, Teller C, Andersen S L, Jensen F B, Hyldig T, Nielsen B. Influence of body temperature on the development of fatigue during prolonged exercise in the heat.  J Appl Physiol. 1999;  86 1032-1039
• 15 Kayser B. Exercise starts and ends in the brain.  Eur J Appl Physiol. 2003;  90 411-419
• 16 Lamb K L. Children's ratings of effort during cycle ergometry: an examination of the validity of two effort-rating scales.  Pediatr Exerc Sci. 1995;  7 407-421
• 17 Marshall W A, Tanner J M. Variations in the pattern of pubertal changes in boys.  Arch Dis Child. 1970;  45 13-23
• 18 Moore F T, Marable S A, Ogden E. Contractility of the heart in abnormal temperatures.  Ann Thorac Surg. 1966;  2 446-450
• 19 Morrison S A, Sleivert G G, Cheung S S. Passive hyperthermia reduces voluntary activation and isometric force production.  Eur J Appl Physiol. 2004;  91 729-736
• 20 Nielsen B, Hyldig T, Bidstrup F, Gonzalez-Alonso J, Christoffersen G R. Brain activity and fatigue during prolonged exercise in the heat.  Pflugers Arch. 2001;  442 41-48
• 21 Nielsen B, Nybo L. Cerebral changes during exercise in the heat.  Sports Med. 2003;  33 1-11
• 22 Nielsen B, Savard G, Richter S A, Hargreaves M, Saltin B. Muscle blood flow and muscle metabolism during exercise and heat stress.  J Appl Physiol. 1990;  69 1040-1046
• 23 Nybo L, Nielsen B. Hyperthermia and central fatigue during prolonged exercise in humans.  J Appl Physiol. 2001;  91 1055-1060
• 24 Rivera Brown A M, Rowland T W, Ramirez-Marrero F, Santacama, Vann A. Exercise tolerance in hot and humid climates: comparison between active, heat-acclimated girls and women.  Med Sci Sports Exerc. 2003;  35 S198 (abstract)
• 25 Rowell L B. Human cardiovascular adjustments to exercise and thermal stress.  Physiol Rev. 1974;  54 75-159
• 26 Rowland T, Blum J. Cardiac dynamics during upright cycle exercise in boys.  Am J Hum Biol. 2000;  12 749-757
• 27 Rowland T, Melanson E L, Popowski B, Ferrone L C. Test-retest reproducibility of maximal cardiac output by Doppler echocardiography.  Am J Cardiol. 1998;  81 1228-1230
• 28 Rowland T, Obert P. Doppler echocardiography for the estimation of cardiac output with exercise.  Sports Med. 2002;  32 973-986
• 29 Savard G, Nielsen B, Laszcynska I, Larsen B E, Saltin B. Muscle blood flow is not reduced in humans during moderate exercise and heat stress.  J Appl Physiol. 1988;  64 649-657
• 30 Tucker R, Rauch L, Harley Y XR, Noakes T D. Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment.  Pflugers Arch. 2004;  448 422-430